Petroleum is found in subterranean formations or reservoirs in which it has accumulated, and recovery of conventional petroleum is achieved by penetrating these reservoirs with wells and permitting the fluid to flow to the surface as a result of natural pressure existing in the reservoir, or by pumping the fluid to the surface as a result of natural pressure existing in the reservoir, or by pumping the fluid to the surface in instances where insufficient natural pressure exists to force it to flow to the surface. There are many reservoirs which contain petroleum too viscous to be pumped from the formation under normal circumstances. When such formations are encountered, production is possible only by means of some process of supplemental recovery, commonly referred to as secondary or tertiary recovery, in which energy is supplied to the formation to force the petroleum to move, and heat and/or a solvent is supplied to the formation to reduce the viscosity of the petroleum so it will flow.
The most extreme examples of formations which contain petroleum too viscous to be recovered by conventional means are the so-called tar sands or bitumen sands, such as those located in the Western United States, Western Canada, and Venezuela. These formations are known to contain huge reserves of bituminous petroleum, but the bituminous petroleum contained therein is too viscous to be recoverable by conventional techniques.
The present state of the art for the recovery of bitumen from tar sand deposits can be generally classified as strip mining or in situ separation. Strip mining requires removal of the overburden by mechanical means and the mixture of bitumen and sand that constitutes the tar sand deposit is then similarly removed by mechanical means and transported to a surface processing plant for separation of bitumen and sand. In situ separation processes make use of techniques for separating the bitumen from the said within the tar sand deposit itself, so the bitumen in some modified form may be transported to the surface with at least a major portion of the sand left in the tar sand deposit. Techniques proposed in the prior art for in situ separation may be classified as thermal or emulsification processes. The thermal techniques include in situ combustion, (fire flooding), and steam flooding. Emulsification processes may also involve the use of steam in addition to an additional chemical to promote emulsification of the high viscosity bitumen so that it may be transported to the surface where the emulsion is resolved into bitumen and water. Although many in situ separation techniques have been proposed in the prior art, none have been both economically and technically successful.
Most known in situ processes involve injection of fluid under fairly high pressures. Injection of high pressure fluid can be conducted safely only is the formation overburden thickness is sufficiently great to contain the high pressure fluids injected thereinto without rupturing. Strip mining of a tar sand deposit is economically feasible only if the ratio of overburden thickness to tar sand deposit thickness is around one or less. Even when the tar sand deposit is fairly shallow, strip mining is still very expensive; the cost of removing overburden and tar sand material represents from 50-60 percent of the total cost of producing a pipeline-acceptable product. Many deposits have overburden which is too thick to permit exploitation by strip mining, and not great enough to contain high pressure fluids for in situ separation processes.
In view of the foregoing, it can be appreciated that there is a substantial, unfulfilled need for a method for recovery of bituminous material from tar sand deposits, particularly those intermediate depth deposits which are not suitable for strip mining or for in situ recovery processes involving injection of a high pressure fluid.